1. Field of the Invention
The present invention relates to a control system for a thermal ink jet printer. Specifically, the present invention relates to an electronically-readable medium relating to the performance of a particular ink-jet printhead, provided directly on a silicon chip forming the printhead.
In thermal ink jet printing, droplets of ink are selectively emitted from a plurality of drop ejectors in a printhead, in accordance with digital instructions, to create a desired image on a surface. The printhead typically comprises a linear array of ejectors for conveying the ink to the sheet. The printhead may move back and forth relative to a surface, for example to print characters, or the linear array may extend across the entire width of a sheet (e.g. a sheet of plain paper) moving relative to the printhead. The ejectors typically comprise capillary channels, or other ink passageways, forming nozzles which are connected to one or more common ink supply manifolds. Ink from the manifold is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated and vaporized by a heating element disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle to the sheet. An exemplary patent showing the general configuration of a typical ink jet printhead is U.S. Pat. No. 4,774,530 to Hawkins.
In most designs of ink jet printing apparatus currently commercially available or contemplated, an essential portion of the printhead, particularly the portion of the printhead having the heating element formed thereon, is in the form of a silicon substrate. This silicon substrate is generally known as the "chip" of the printhead, and typically includes not only the heating elements formed thereon, but the series of electrical leads connecting each of the heating elements to a voltage source. The leads are typically in the form of a pattern of aluminum depositions, and a typical construction of the heating element is in the form of a deposit of polycrystalline silicon which forms an element having a predetermined resistance.
In a common method of manufacture of thermal ink-jet printhead modules or "chips," each chip is sized to accommodate 128 nozzles spaced at a density of 300 nozzles per inch; in terms of a chip, 128 heating elements are provided, each heating element having at least one lead connected thereto, as well as any other electronic circuitry which may be formed on the chip. In mass production of such chips, as many as 200 or more chips may be formed in a single silicon "wafer," the entire wafer being manufactured in one step and then subsequently cut, or "diced," into the chips themselves.
An important practical concern for commercially-acceptable printheads is a consistency of performance characteristics among a plurality of chips. An important factor affecting the quality of an image formed on a sheet is uniformity of "spot size." All of the ejectors in a printhead must create spots on the sheet of uniform size given certain operating conditions, particularly power to the heating elements and temperature of the liquid ink. Equally important is spot size uniformity among various printhead chips. In many commercially available or contemplated thermal ink jet printing apparatus, it is intended that the chips will have an expected life shorter than that of the machine in general, and therefore will require periodic replacement. Other known designs include a plurality of separate, abutting chips being used simultaneously. Various manufacturing conditions may cause variations among chips which may be very small in absolute terms, but which will have a significant effect on spot size uniformity. Minute variations in, for example, the dimensions of the channels forming the nozzles, or in the resistivity of the polysilicon forming the heating elements, may have a substantial effect on the spot size associated with a particular chip. For customer satisfaction, it is necessary that spot size remain uniform not only under various external conditions for one chip, but also from chip to chip as chips are replaced during the course of the life of the machine. Such chip-to-chip uniformity is particularly crucial in an apparatus in which multiple chips are employed simultaneously, as in a full-width array apparatus in which as many as 20 chips may be abutted to form an array which extends across a sheet. It is therefore desirable that a thermal ink-jet printing apparatus include provision for recognizing variations in the performance characteristics of individual chips, and further to be able to compensate for these variations.
2. Description of the Prior Art
European Patent Application A2-0 412 459 discloses one arrangement by which information for controlling the driving conditions of a printhead is stored on a medium which forms part of the ink cartridge, in an apparatus wherein an individual printhead is associated with an expendable ink cartridge. The medium may contain information such as the color of the ink in the ink cartridge. In various described embodiments, this medium may be in the form of a predetermined resistance located on the cartridge, a magnetic stripe, an optically readable bar code, or some three-dimensional information pattern disposed on the housing of the ink cartridge.
German Patent Application P-4,020,885 discloses an ink-jet printhead wherein the chip associated therewith includes an additional reference resistor, created at the same time as the heating resistors, and which therefore should share many of the relevant electrical characteristics with the actual heating resistors. The provision of the reference resistor enables the printing apparatus to pre-test the chip so that an optimum pulse voltage may be set for the heating resistors, in correspondence with the measured resistance value.